Kit Car Push Rod Suspension

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The Push Rod rear suspension is now being mocked up – ready for assembly. The rear hubs will be removed a welded on a jig as will the upper and lower arms.

Although much work has been done; powder coating parts, assembling the brand new c.v. joints, shafts and hubs; not much creative work has happened.

For the rear hubs, I priced up the steel I needed. There was no problem in finding local stockist for the steel, the problem was getting small enough lengths. I am using the same 5″ diameter x 65mm long tubes as the standard Haynes Roadster (Tube RU5). Therefore, I was looking for a length a little long than 130mm; not long. The shortest my very friendly and helpful stockist would part with, was 6 metres long for £22. I then needed some 10mm thick plate to make the plates (RU7 and RU8) to mount the brand spanking new Granada rear hubs I’d acquired. A 60mile round trip to a surplus yard would yield this for around £20. So, for around  £42, I could get the steel I needed.

These parts would take many hours of filing and drilling to complete. Ideally, the 5″ tubes would need finishing in a lathe to get them square (further cost). I would also need a new tap to cut the threads as mine seemed to be missing. Can you see my shoping list and timescales going off the chart? I could.

Option 2:

I could buy the parts already made and this is what I did. I bought them from Dave Oxley at Rogue Engineering.

Haynes Roadster Rear Hub - Rogue Engineering

They arrived; well packed; 1½ days after clicking the ‘Buy Now’ button. They were very well finished and were as accurate as the ‘laser cut’ description suggests. I had bought a complete kit of parts needed for the standard Haynes Roadster uprights as a couple of the other tubes would also come in handy. The cost? £55. I still had all my knuckles, I didn’t have a workshop full of surplus steel and I didn’t even have to sweep up heaps of steel filings. BARGAIN 🙂

I know I want to build every single millimetre of this car myself, but sometimes letting somebody else lend a hand makes perfect sense. In fact many of the parts used on this build will be straight out of a catalogue e.g. the threaded tube inserts, the urethane assemblies complete with stainless steel crush tubes, the rose jointed turn buckle push rods etc. One thing I noticed during my web ramblings was that imperial threaded items (UNF) were significantly cheaper than their metric cousins. This could well be due to the strength of the American Hot Rod Scene. In the US there is ‘pre-packed’ access to high quality components that we have trouble sourcing and they are at prices significantly lower than in the UK. Simply as an example why not browse http://www.speedwaymotors.com/. Many of the components would be at home on a ‘trailor queen’ show car, yet they are at prices only large scale, mass production brings.

Cantilever Push Rod Rear Suspension

The Sierra Cosworth rear axle employs push rod cantilevers and horizontal in-board coil-over shock absorbers. Unlike many I have seen recently, this system will give a rising spring rate during bounce and roll. Although the Suzuki Drz400 off road motorbike rear shocks are somewhat on the large and chunky size, they are fitted as standard with 220lbs springs. With a cantilever set-up like this, spring rate seen at the wheel rises during compression. At normal ride height the spring rate seen at the wheel is around 180lbs/inch, giving a soft ride, however as the wheel rises spring rate rises logarithmically to over 600lbs/inch. Some argue that this negates the need for a anti-roll bar; I can see the logic; but I have left room for one just in case.

The arms will employ clevis arrangements. The ones shown are for illustration purpose only. I want to have a small degree of adjust-ability but I don’t want to use rose joints. I want to use standard polyurethane rods ends. This should hopefully make the ride a little less harsh.

The push-rods are turn-buckle arrangements, that have opposite threaded rose joints at each end. In conjunction with the triple adjustable coil-over shocks and the multi-position, top in-board shock mount, I can adjust spring rate and keep ride height un-changed. I can also change the rate that spring rates rise and fall e.g. a sharp rise in spring rate or a much more gentle one. (Note in the above picture only one turn-buckle link is shown).

Turn-buckle linkage

The Suzuki DrZ400 also supplies the rather pretty aluminium bell-cranks cantilevers.

Bell Crank Cantilevers

Drz400 shock absorber

I didn’t use the standard Suzuki push rods as I want toe in/out adjustment in the suspension. Without the rose jointed ends this would not be possible.

Using motorbike coil-overs has the following advantages:

  • They are cheap as chips (I have ones with show-room only mileage and they cost about £10 each)
  • They are readily available
  • They often come with lovely aluminium cantilevers complete with roller bearings.
  • Riders often change them as soon as they buy a bike.
  • The small road bike versions are very light

Disadvantages:

  • The off road versions and large
  • The road (sport bike)  versions have very high spring rates (often approaching 700lbs)
  • The smaller the shock and the larger capacity the donor motorbike, the greater chance there is of needing custom coil springs (closer to 220lbs).

For more information on push rod rear suspension click here

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